Generally, a rear subframe (having the same meaning as “rear suspension cross member”) for supporting a rear has comprises right and left side member segments and a cross member segment coupling the side member segments together. As an automotive rear vehicle body structure comprising such a rear subframe, there have been known various structures as illustrated in FIGS. 16 and 17.
A conventional structure (Patent Document 1) schematically illustrated in FIG. 16 in the form of a bottom view comprises a subframe 80 having right and left side member segments 81, 81 and front and rear cross member segments 82, 83, wherein front and rear fixing sections 81a, 81b provided in respective front and rear regions of each of the right and left side member segments 81, 81 are coupled to a respective one of right and left rear side frames 84, 84, and a front portion 81c of each of the right and left side member segments 81, 81 is coupled to a vehicle-body cross member 85 through a respective one of extension members 86, 86.
In FIG. 16, the side member segment 81 is curved inwardly in a vehicle width direction to have an approximately circular arc shape, in top plan view. This curved structure is intended to ensure an installation space for a suspension spring (coil spring) in order to support the suspension spring by a base of the rear side frame 84, and to increase an arm length (a length of an arm 88) to suppress a change in suspension geometry (change in toe angle, camber angle, etc.) due to an up-and-down movement of a rear wheel 87.
In FIG. 16, the reference code 88 indicates a front lower arm, and the reference code 89 indicates a rear lower arm. Further, in the figures, the arrowed line F indicates a vehicle forward direction, and the arrowed line R indicates a vehicle rearward direction.
The conventional structure illustrated in FIG. 16 has the following problems.
First, a distance between the front and rear fixing sections 81a, 81b of the side member segment 81 is relatively long and curved. Thus, in order to ensure rigidity against an input of lateral force, it is necessary to increase a plate thickness of the side member segment 81, resulting in an increase in weight.
Further, in the event of a rear collision, a rear impact load received by a rear region of the rear side frame 84 is concentrated on a front region of the rear side frame 84, as indicated by the arrowed line in FIG. 16. This causes a problem of an increase in amount of deformation in members around a side sill located forward of the rear side frame 84.
Besides, a distance between the front fixing section 81a and a support section 88a for the front lower arm 88 is relatively long, which is disadvantageous in terms of rigidity.
Moreover, the front portion 81c of the side member segment 81 is connected to the vehicle-body cross member 85 through the extension member 86, so that rigidity of the subframe 80 is enhanced somewhat, but, on the other hand, an increase in length of the extension member 86 gives rise to a problem of causing the extension member 86 to more easily undergo deformation, and of increases in required installation space and weight of the extension member 86.
A conventional structure (Patent Document 2) schematically illustrated in FIG. 17 in the form of a bottom view comprises a subframe 80A having right and left side member segments 81, 81 and front and rear cross member segments 82, 83, wherein front and rear fixing sections 81a, 81b provided in respective front and rear regions of each of the right and left side member segments 81, 81 are coupled to a respective one of right and left rear side frames 84, 84. Further, the side member segment 81 is formed to be curved inwardly in a vehicle width direction to have an approximately circular arc shape, in top plan view, for the same purpose as that of the conventional structure in FIG. 16.
In FIG. 17, each of the side member segments 81, 81 is formed to be curved inwardly in the vehicle width direction, and the front fixing section 81a is coupled to the rear side frame 84. Thus, a front region of the subframe 80A is strongly supported by the rear side frame 84 which is a vehicle-body strength member extending in a front-rear direction, and the side member segment 8 becomes more likely to undergo deformation. This has been considered to be desirable in terms of coupling strength between the subframe 80A and a vehicle body and further in terms of suppression of a forward displacement of the subframe 80A during a rear collision.
However, as with the conventional structure in FIG. 16, the conventional structure illustrated in FIG. 17 is incapable of shortening a distance between the front fixing section 81a and a support section 88a for a front lower arm 88, which is disadvantageous in terms of rigidity.
As above, except for the point about the extension member 86, the conventional structure illustrated in FIG. 17 has the same problems as those in the conventional structure in FIG. 16. In FIG. 17, the same element or component as that in FIG. 16 is assigned with the same reference numeral or code.
Additionally, the conventional structure illustrated in FIG. 17 is configured to arrange the support section 88a for the front lower arm 88 and a support section for a front upper arm (not illustrated) provided upward of the front lower arm 88, at the same position in the front-rear direction, to cancel out loads between the upper arm on an upper-right side and the lower arm on an lower-right side and between the upper arm on an upper-left side and the lower arm on a lower-right side, i.e., by means of a cross (X) arrangement in front view. In this way, same-phase lateral forces (lateral forces causing the right and left rear wheels 87 to tilt toward the same side) input during turning of an automotive vehicle or the like can be cancelled out.
Although the conventional structure illustrated in FIG. 17 can cancel out same-phase lateral forces, it has the following problem, with regard to different-phase lateral forces (lateral forces causing the right and left rear wheels 87, 87 to tilt toward respective different sides) to be input during traveling of the vehicle on bumpy road or the like. In the conventional structure illustrated in FIG. 17, the cross member segment 82 is made from a panel, and an amount of vertical offset between the support section 88a of the front lower arm 88 and a central portion of the cross member segment 82, so that there is a problem that it is unable to sufficiently ensure rigidity against different-phase lateral forces, and the cross member segment 82 made from a panel undergoes plane resonance at a frequency of about 300 Hz.
As above, the conventional automotive rear vehicle body structures fail to sufficiently satisfy both ensuring of rigidity against same-phase lateral forces and different-phase lateral forces, and suppression of plane resonance.